In a data distribution system from a central site to a great number of users, the organisers of such a resource are regularly occupied with two matters. The first concerns the security of the data, that is, to ensure that the targeted user is the only one to receive the requested data, and for his/her use only.
The second matter is the capacity of the distribution means, that is, the galvanic or optic lines, cable, or the radio-relay network.
The operating center has an important data base that contains the products offered to the users either subject to payment or free of charge. According to the recent structures of this type of center only one part of these products are stored locally, the other products being stored by the producer of these data and linked to the operating center by high speed communication means.
The user sends a request to the operating center comprising his/her identity, address, and the desired product.
The operating center determines the acceptability of this request, particularly according to the payment methods of this user. These methods can be by subscription, with unlimited access to the products, upon presentation of bills, payment by credit card, or pre-payment.
The product is sent in encrypted form with a user's key, that is, according to a key that is contained in a security module at the user's location.
This type of service is used for hiring video films through pay television. In this use the subscriber has a security module connected to his/her decoder that processes the encrypted data and sends the decryption keys to the decoder.
This security module also administers credit information.
This system works today with the so called form NVOD (Near Video On Demand) and VOD. In the first case (NVOD) the proposed product for buying is sent loopback to all the users, sometimes through several channels simultaneously with a time delay. The buyer then can only buy this specific product and will wait for the next emission to start the visual display. An embodiment of this mode consists in sending several products during the time of inactivity of the system (generally in the morning), said products being stored in the mass memory of the decoder. The products thus memorised are then proposed to the user for buying and can be visualised at any given moment.
In either of these embodiments the number of proposed products is lower than ten, taking into account the transmission and storage possibilities of a product.
The second case, which is particularly interesting, proposes supplying the product of your choice amongst a very important catalogue of products. Each user can then choose from this catalogue and the operating center sends him/her the desired product. This is only possible in a restricted environment with very important transmission capacities.
The implementation on a large scale of such a system has very quickly collided with the bandwidth limits of the transmission means. Just to remember, it is reminded here that the transmission of a 2 hour film represents 1 gigabyte of high compressed data.